A conventional self-propelled truck or bogie truck shown in FIG. 11 is well-known, which truck has a base portion C a plurality of non-driving wheels A are attached to a lower face whereof, an elevating plate E rotatably journalled to the base portion C through a shaft D, driving wheels G journalled to the elevating plate E through a shaft F, a spring H placed between the elevating plate E and the base portion C so as to press down the elevating plate E, a motor I rotating the elevating plate E against resilient force of the spring H around the shaft D so as to move the driving wheels G from their lower running position in which the driving wheels G contact with the floor face B to their upper storage position in which they aren't contact with the floor face B, and a running sensor detecting a guide tape arranged on the floor face B.
When the elevating plate and the driving wheels move to the lower running position, the spring in the conventional system above extends and gives a predetermined spring or resilient force to the driving wheels as a ground contacting pressure. On the contrary, when the driving wheels move to the upper storage position, the spring shortens and accordingly it is necessary to use a force shortening the spring which is stronger than the ground contacting pressure. As a result, a motor for elevating the driving wheels necessitates a strong force compressing the spring, which force is stronger than the ground contacting pressure.